Night Rocksters will spawn on the surface in dark places usually only at night, but they can also spawn during the day in the darkness of Caves and throughout the Fossil layer underground on rocks like Bedrock and Limestone.

Occasionally they might come out of Caves during the day to hunt player characters if they can sense / see them. They do not easily spawn on artificial player-made Biomes built from Bedrock and/or Limestone.

Like red daylight Rocksters, Night Rocksters have the ability to hunker down and block all incoming damage, an effect that lasts for ~4 seconds and is ended with a powerful spin-attack capable of sending the player flying.

With update R41 the hitpoints of Night Rocksters have been reduced, however their armor (defense) has been increased, which makes Armor-Piercing Bombs more efficient against them now.

According to Playful, Night Rocksters have 150 health points and based on tests, to be killed they require:

You cannot hurt Night Rocksters while they cover themselves with their claws. Every hit that you deal to Night Rocksters while they are blocking will only reduce the durability of your weapon, not inflict any damage to the Night Rocksters.